Method and apparatus for controlling the alloying of zinc coatings



METHOD AND APPARATUS FOR CONTROLLING THE ALLOYING OF ZINC COATINGS FiledSept. 5, .1963

I March 67 P. E. SCHNEDLER 3,

INVENTOR PAUL E. SCHNEDLER,

1- BY h1g3 ATTORNEYS United States Patent Office 3,307,968- PatentedMar. 7, 1967 3,307,968 METHOD AND APPARATUS FOR CONTROLLING THE ALLQYTNGF ZlNC CUATINGS Paul E. Schnedler, Middletown, Ohio, assignor to ArmcoSteel Corporation, Middletown, Ohio, a corporation of Ohio Filed Sept.3, 1963, Ser. No. 3%,123 8 Claims. (Cl. 117-114) .bination of the twosuch that the alloying of the zinc with the base metal of the stripwould proceed in a more uniform manner and produce a more completelyalloyed coating exhibiting a one phase alloy.

The reason alloyed coatings have come into demand in recent times isthat they do not present a spangled appearance but a dull surfacetexture which readily takes paint and is advantageous for variousspecific purposes. In order to obtain the maximum utility for an alloyedzinc coated steel body the alloying must be as uniform as possible. Thestrip must be completely alloyed over its entire surface and no brightunalloyed regions must remain; at the same time over-alloying must beavoided.

One of the basic features of the aforesaid patent was the disclosure ofheating the strip from within electrically. The advantages of heatingfrom within result from the fact that areas which have completelyalloyed have a high emissivity and they will radiate heat rapidly andthus tend to cool. Adjacent regions which have not alloyed and stillhave a bright surface with a low emissivity will retain the heat so thatalloying in these regions will continue to take place. Thus when thestrip is heated from within, the phenomena of radiation from bright anddull surfaces produce a self-leveling effect to enhance the uniformityof alloying. When the strip is heated externally, of course thesephenomena operate in conflict with each other and accentuate theunevenness of alloying.

It is therefore one of the objects of the present invention to providefor a very accurate control of internal heating and to do this with aminimum of energy input. In this connection it is another object of theinvention to provide for heating the strip just after it emerges fromthe coating bath from the temperature at which it then is up to atemperature at which alloying proceeds rapidly. This means that theelectrical heating from within may be more accurately controlled.

In conection with the control of the heating apparatus, it is anotherobject of the invention to provide sensing means for sensing the degreeand extent of alloying the strip surface. This sensing is based upon thefact that unalloyed coating will be highly directionally reflectivewhile alloyed coating will not be highly directionally reflective.

It is another object of the invention to utilize photosensitive devicesto sense the reflectivity of the coating and to arrange thesephotosensitive devices in a novel way such that they will be moregreatly affected by dispersed light rays reflected from a dull area thanby bright rays reflected from a bright surface.

It is still a further object of the invention to provide a plurality ofsuch sensing devices in a row transversely of the strip and to associatewith these sensing devices a plurality of supplemental heaters such thatif an element of the strip at a particular distance from one edge is notsubstantially the same distance from the same edge will be energized toincrease the heat put into the strip, and thereby increase the rate ofalloying.

These and other objects of the invention which will be pointed out ingreater detail hereinafter, or which will become apparent as thedescription proceeds, are accomplished by that series of method stepsand by that construction and arrangement of parts of which the followingwill describe an exemplary embodiment.

Reference is made to the drawings forming a part hereof and in which: i

FIG. 1 is a diagrammatic cross-sectional view through a portion of acoating line.

FIG. 2 is a fragmentary elevation thereof as seen from the right of FIG.1; and

FIG. 3 is a diagram useful in understanding the interaction of theheating devices.

Briefly, in the practice of the invention there are provided threeseparate heating means. As the strip emerges from the coating bath itpasses through a high intensity booster heater which raises thetemperature of the strip to optimum alloying temperature. Since thealloying reaction is a diffusion reaction, it is necessary to maintainthe strip at alloying temperature for a period of time necessary tocomplete the reaction. The second heating step involves electricalresistance heating from within to maintain the strip at the optimumalloying temperature for the period of time necessary to permit thereaction to go to completion. Sensing devices are provided to determinewhen alloying is complete and to energize the electrical heating meanswhen alloying is not complete.

The third heating means involves a plurality of heating elementsdisposed transversely of the strip and which may be selectivelyenergized to heat any element of the strip across the width thereof.These selective supplemental heating devices are controlled byphotosensitive devices correspondingly positioned transversely of thestrip so that if, for example, a portion of the strip at a distance ofsay 12 inches from one edge is not fully alloyed, supplemental heat isadded at a point 12 inches from that edge of the strip.

The longitudinal areas of unalloyed coating may result from a number ofcauses: generally the coating will tend to be heavier at the stripedges, so that the alloying rate along the edges will be different thanin the center; and uneven wear on the exit rolls may cause longitudinalareas of the strip passing over points of greater wear to have a heaviercoating, which will require more heat or time for complete alloying.Variations such as those just described are common, and well Withincomercial tolerances.

The sensing devices are arranged in two sets; one set to sense dullareas of the strip at the point where the strip should be bright, andthe other set arranged to sense bright areas of the strip at a pointwhere alloying should be complete. The light sources and photocells ofthe two sets are arranged differently as will be described hereinafterto sense, respectively, dull areas and bright areas.

Referring now in more detail to the drawings, there is shown at 11) acoating pot containing molten zinc indicated at 11. A strip 12, whichhas been subjected to any desired pretreatment, passes around ahold-down roll 13 within the pot and thence upwardly through exit rolls14. The coated strip 12a then passes upwardly about a turning roll 15and thence to storage, coiling or other processing.

At 16 there is shown diagrammatically a high intensity booster heater.This heater may be either localized electrical induction or highintensity radiation or a flame type gas heater. The object of the heater16 is to heat the strip to optimum alloying temperature as soon aspossible after leaving the coating bath. Optimum alloying temperaturefor aluminum-bearing zinc is approximately 925 to 1050 F., although asatisfactory range would extend from about 900 F. to about 1250 F. Itshould be noted in this connection that since the surface of the freshlycoated strip with the molten coating on it is uniformly bright, externalheat may be used to raise the temperature of the strip uniformly andthis is no contradiction of the principles described in the aforesaidPatent No. 2,986,808. The principles of that patent come into effectwhen alloying has started; and after alloying has Started, if the heatis applied externally, the uniformity of the alloying rate becomesprogressively worse, While if the strip is then heated from within, theself-leveling effect above mentioned comes into play.

The heating means 16 regardless of what type is being used, iscontrolled by a radiation pyrometer or infra-red detection-deviceindicated at 17.

At 18 there is disclosed the second heating device, which producesheating from within. In the particular embodiment illustrated this is aninductive resistance heating apparatus and the element 18 is atransformer primary and the entire length of strip 12a from the exitrolls to the roll 15, coupled with a return conductor 22, constitutesthe secondary and is thus heated. This heat must be very accuratelycontrolled because the maintenance of alloying temperature is the mostcritical feature of the alloying operation. In theory, the currentflowing through the strip 12a produces a variable controlled coolingeffect so that areas of the strip which have completely alloyed and havea high emissivity will radiate heat faster than heat is being suppliedinternally and actually become cooler; while adjacent regions which havenot alloyed and still have a bright surface with a correspondingly lowemissivity will retain the heat, and alloying in these regions willcontinue to take place. It will also be noted that because the boosterheater 16 has heated the strip up to alloying temperature, the onlyrequirement of the heating device 18 is to maintain the strip atalloying temperature or to control cooling rate to achieve the properdegree of alloying.

To control the action of the heating device 18 there are provided twoseries of photocells, the first indicated at 19, and the second at 26.As best seen in FIG. 2, there are a plurality of photocells 19 and aplurality of photocells 20. It will be apparent that the photocells inthe row 19 will be positioned to monitor the strip when it is normallystill bright; while the photocells 20 are positioned to monitor thestrip when it is normally completely alloyed. Variations from normalconditions are indicated by the two series of photocells and, throughsuitable circuitry operate the induction heating apparatus 18. Theseconnections do not form a part of the present invention and are withinthe skill of a competent electrical engineer.

A third heating apparatus is provided and this involves a plurality ofselective heating devices 21. These are spaced in a row transversely ofthe strip in positions corresponding to the photocells 19 and 20. Theheaters 21 may be either combustion type or electric heaters, and theirpurpose is to provide supplemental heat to particular areas transverselyof the strip which are not alloying at a sufliciently fast rate. Theindividual heaters 21 are controlled by a pair of photocells 19 and 2%By examination of FIG. 2 for example, it can be seen that the leftmostpair of photocells 19 and 20 are aligned with the left-most heater 21,and that correspondingly there is a heater 21 for each additional pairof photocells 19 and 20 all the way across the strip. Again, theelectrical connections or control circuits have not been shown becausethey are within the skill of a competent engineer.

The disposition of the photocells for proper results is an importantfeature of this invention. A photocell is normally thought of as beingresponsive to reflected light and to some degree in proportion tointensity of the reflected light beam. In the present case, it must beborne in mind that when the upwardly moving strip reaches the positionof the bank of photocells 19, it is predominantly unalloyed andtherefore bright. It is possible, however, that small areas of the stripmay have proceeded to alloying and may, therefore, show a dull surface.If the photocells 19 were arranged to respond to bright light, then asmall dull area would not produce a signal of significant value whichwould be useful for control purposes. Similarly and conversely by thetime the strip reaches the bank of photocells 20, it is predominantlyalloyed and therefore dull. Here again it is possible that a small areaof the strip may not be completely alloyed and will therefore be brightand if the photocells of the bank 20 were designed to be responsive todull light, one small area of brightness would not give a significantsignal.

Therefore, the photocells in the bank 19 are arranged to be responsiveto the presence of dull areas in a bright strip and those on the bank 20are arranged to be responsive to small areas of brightness in apredominantly dull strip.

This is accomplished as shown in FIG. 1. The photocells 20 and theirassociated light sources 20a are dis posed so that the bisector of theangle between the incident light from the source 20a and the reflectedlight impinging on the photocell Ztl is normal to the strip. Under thesecircumstances, the dull strip will not affect the photocell 21) but asmall bright area resulting from incomplete alloying will produce areflection which will cause the photocell 20 to put out a signal ofsignificant value. In order to eliminate errors caused by erratic stripmovement such as buckling and waving, the strip is caused to pass over ahilly roll 15a and the photocell and the light source are aimed at thestrip at the point where it passes over the billy roll 15a.

On the other hand, the photocells 19 and their associated light sources19a are positioned so that the bisector of the included angle is notnormal to the strip. The strip at this point is normally bright and thelight beam from the source will not enter the photocell 19 since theangle of reflection is equal to the angle of incidence. The photocellmust be positioned so that small buckles and waves in the strip will notcause the bright reflection to enter the photocell. However, the beamfrom the source 1942 reflected from the dull areas of the strip isbroken up and scattered as indicated by the solid arrows in Fig. l andsome portions of this reflected light from the dull areas will enter thephotocell 19 as shown in the drawing. In this way a dull area in apredominantly bright strip will affect the photocell 19 to produce asignal of significant value.

It has been found experimentally that if a light source 19a and itsassociated photocell 19 are located as close together as possible, withthe light beam and the optical axis of the photocell substantiallyparallel, the most significant difference in reflectance as between adull and a bright surface will be obtained if the light beam and theoptical axis of the photocell (or their bisector, if they are notparallel) are disposed at an angle of about 25 to the normal to thestrip surface. It is at this angle that the difference in reflectancebetween dull and bright surfaces is greatest, and the amount of thisdifference is only slightly affected by slight strip movement, whichwould, in effect, change this angle.

In Fig. 2 there is diagrammatically indicated the way in which thesignals from the photocells may operate. So long as, in any given pairof photocells 19 and 20, the photocell 19 is not putting out a changesignal because no dull spots are being encountered, and so long as thephotocell Ztl is not putting out a change signal because no bright spotsare encountered, it means that the strip passing that particular pair ofphotocells 19-20 is unalloyed at the point 19 and fully alloyed at thepoint 20. This means that the heat input from the various heaters isdoing the job correctly.

If now, for orie reason or another, the strip as it passes the photocell19 is dull, it means that alloying has already started and a signal toreduce heat will be put out by the photocell 19. On the other hand, ifthe strip passing the photocell 20 has a bright area indicating thatalloying is incomplete, then the photocell 20 will put out a signal toincrease heat, and hence alloying. The signals from the photocell pair1920 are transmitted to the individual heater 21 which is aligned withthe photocell pair 19-20. If the signal to the heaters 21 has come fromthe photocell 19, the input to the respective heater 21 is reducedwhereas if the signal comes from the photocell 20, then the input to therespective heater 21 is increased. In this way uniformity transverselyof the strip with regard to alloying process is achieved. Since,however, the heaters 21 are supplemental heaters, a relationship isestablished between their summation and the induction heating device 18.Thus, in FIG. 2 an input to the various heaters 21 is shown at 30. Thisinput is indicated as entering a kilowatt meter 31 and thence going tothe individual heaters 21 subject to the control by the signals from thephotocells 19 or 20. It will be clear that the meter 31 reads the totalinput to all of the heaters 21. It is desired to keep the input to thevarious heaters 21 more or less in the middle or a range between minimumand maximum wattage. This range may be indicated diagrammatically by theportion 32 of the meter 31 and so long as the input remains within therange 32, no signal is transmitted to the heater 18. However, if theseveral pairs of photocells each reading on different portions of thestrip cause the total input to the various heaters 21 to rise above apredetermined maximum, then a signal is sent to the heating member 18 toincrease the power input 35 to the member 18. This is indicated by thearrow 33 accompanied by the plus sign. If, on the other hand, thesignals from the pairs of photocells 19 and 20 cause the various heaters21 to be cut off to the point where the total input to them is below therange indicated at 32, then a signal is sent as indicated by the arrow34, accompanied by the minus sign, to cut down the input 35 to theheater 18. In this manner, the input to the various heaters 21 ismaintained within the range indicated at 32 and preferably at about 50%of capacity.

It will be understood that numerous modifications may be made withoutdeparting from the spirit of the invention and no limitation is intendedother than as specifically set forth in the claims which follow.

The invention having now been fully described, what is claimed is:

1. In a coating apparatus for coating a strip of metal with a layer ofanother metal, wherein the strip passes through a bath of molten coatingmetal and then upwardly vertically in an alloying run; booster heatingmeans positioned just above said bath to heat said strip to atemperature at which alloying of said coating metal with the metal ofsaid strip will take place, electrical means for supplying heat to thestrip from within over the entire alloying run to maintain alloyingtemperature in the strip during the alloying reaction, a plurality ofsupplemental heating devices immediately above said booster heatingmeans to provide heat to longitudinal zones of the strip selectively,and a plurality of control means responsive to the reflectivity of thestrip in said longitudinal zones disposed above said supplementalheating devices and in the latter portion of said alloying run forcontrolling respective ones of said supplemental heating devices.

2. Apparatus according to claim 1, wherein said control means includes abank of light-sensitive devices and associated light sources positionedadjacent the end of said alloying run at a point in the strip travelwhere alloying should be complete, and the strip, therefore, uniformlydull, said light-sensitive devices and associated light sources beingaimed at the strip such that the bisector of the angle between theincident light rays from said light sources and reflected light raysimpinging upon said lightsensitive devices is substantially normal tothe strip surface, whereby small bright areas in the generally dullsurface will cause said light sensitive devices to give a control signalof significant proportion.

3. Apparatus according to claim 2, wherein said control means includesan additional bank of light-sensitive devices and associated lightsources positioned in the latter part of said alloying run at a point inthe strip travel where alloying should not have started, and the strip,therefore, should be uniformly bright, said light-sensitive devices andassociated light sources being aimed at the strip such that the bisectorof the angle between the incident light rays from said sources and theirreflection impinging upon said light-sensitive devices is at an angledifferent than to the strip surface, whereby small dull areas in thegenerally bright surface will cause said light-sensitive devices to givea control signal of significant proportion.

4. Apparatus according to claim 3, wherein said angle is about 25 to thenormal to the strip surface.

5. Apparatus according to claim 1, wherein means are provided to measurethe total power input to said supplemental heating devices and meansoperative when the total input to said supplemental heating devicesexceeds a predetermined value to increase the power input to saidelectrical means, and means operative when the total power input to saidsupplemental heating devices drops below a predetermined point to reducethe power input to said electrical means.

6. The method of producing on a metallic strip a highly uniform, onephase, alloyed zinc coating, wherein molten zinc is applied to the stripby passing the strip through a bath of molten zinc and thence verticallyupwardly in an alloying run; which includes the steps of rapidly heatingsaid strip to alloying temperature as it leaves said coating bath,maintaining said strip at alloying temperature by electrically heatingsaid strip from within throughout said alloying run, sensing thereflectivity of longitudinal zones of the strip adjacent the end of saidalloying run at a point at which alloying should be complete, andcausing sensings indicating high reflectivity, and therefore incompletealloying, in a particular longitudinal zone to cause supplementalheating of said particular longitudinal zone after said rapid heatingand before said sensing.

7. The method of producing on a metallic strip a highly uniform onephase alloyed zinc coating, wherein molten Zinc is applied to the stripby passing the strip through a bath of molten zinc and thence verticallyupwardly in an alloying run; which includes the steps of rapidly heatingsaid strip to alloying temperature as it leaves said coatingbath,maintaining said strip at alloying temperature by electrically heatingsaid strip from within throughout said alloying run, supplementaryheating applying to longitudinal zones of said strip immediately aftersaid rapid heating, sensing the reflectivity of said zones adjacent theend of said alloying run at points at which alloying should be complete,causing sensings indicating high reflectivity, and therefore incompletealloying, in a particular one of said zones to increase the applicationof supplementary heating in said zone, sensing the reflectivity of saidzones in the latter port of said alloying run at points at whichalloying should not have started, and causing sensings indicating lowreflectivity, and therefore incipient alloying, in a particular one ofsaid zones to decrease the application of supplementary heating in saidzone.

8. The method of claim 7, which includes the step of measuring the totalamount of supplementary heating supplied to said longitudinal zones,increasing the electrical heating of the strip from within when saidtotal supplementary heating exceeds a predetermined value and decreasingsaid electrical heating from within when said total supplementaryheating drops below a predetermined value.

7 8 References Cited by the Examiner OTHER REFERENCES UNITED STATESPATENTS Burgwin, S. L.: Electronic Regulation of Industrial t 17 6 61922188 8/1933 zworykint jPurgseslsgzhln Instrumen s, pp 328 330, 3 9,2,947,212 8/1960 Woods 8814 5 2 9 0 19 1 Schmdler 117 114 RALPH S.KENDALL, Przmary Examiner.

3,058,840 10/1962 Kerr et al 118-620 X J. R. BATTEN, JR., AssistantExaminer.

1. IN A COATING APPARATUS FOR COATING A STRIP OF METAL WITH A LAYER OFANOTHER METAL, WHEREIN THE STRIP PASSES THROUGH A BATH OF MOLTEN COATINGMETAL AND THEN UPWARDLY VERTICALLY IN AN ALLOYING RUN; BOOSTER HEATINGMEANS POSITIONED JUST ABOVE SAID BATH TO HEAT SAID STRIP TO ATEMPERATURE AT WHICH ALLOYING OF SAID COATING METAL WITH THE METAL OFSAID STRIP WILL TAKE PLACE, ELECTRICAL MEANS FOR SUPPLYING HEAT TO THESTRIP FROM WITHIN OVER THE ENTIRE ALLOYING RUN TO MAINTAIN ALLOYINGTEMPERATYRE IN THE STRIP DURING THE ALLOYING REACTION, A LURALITY OFSUPPLEMENTAL HEATING DEVICES IMMEDIATELY ABOVE SAID BOOSTER HEATINGMEANS TO PROVIDE HEAT TO LONGITUDINAL ZONES OF THE STRIP SELECTIVELY,AND A PLURALITY OF CONTROL MEANS RESPONSIVE TO THE REFLECTIVITY OF THESTRIP IN SAID LONGITUDINAL ZONES DISPOSED ABOVE SAID SUPPLEMENTALHEATING DEVICES AND IN THE LATTER PORTION OF SAID ALLOYING RUN FORCONTROLLING RESPECTIVE ONES OF SAID SUPPLEMENTAL HEATING DEVICES.
 7. THEMETHOD OF PRODUCING ON A METALLIC STRIP A HIGHLY UNIFORM ONE PHASEALLOYED ZINC COATING, WHEREIN MOLTEN ZINC IS APPLIED TO THE STRIP BYPASSING THE STRIP THROUGH A BATH OF MOLTEN ZINC AND THENCE VERTICALLYUPWARDLY IN